A non‐linear superconducting transmission line has been successfully employed in heterodyne detection of sub‐millimeter waves. In our experiments an Nb/Al/AlOx/Nb tunnel junction, measuring 0.15×40 &mgr;m, with a critical current density of about 2500 A/cm2is used as a distributed mixer element. We have demonstrated that quantum limited sensitivity can be achieved with this type of device. At 460 GHz, aY‐factor of 2.5 has been recorded, corresponding to a double‐side‐band receiver noise temperature of 80 K. The double‐side‐band conversion loss is about 1 dB and the mixer noise temperature is estimated to be 18(±10) K. ©1995 American Institute of Physics.